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Full text of "The Osteology of Mesohippus barbouri"

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Bulletin of the Museum of Comparative Zoology at Harvard College. 

Cambridge, Mass. :The Museum, 
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Bulletin of the Museum of Comparative Zoology 

AT HARVARD COLLEGE 

Vol, LXXII, No. 11 



THE OSTEOLOGY OF MESOHIPPUS BARBOURI 



By Eeich M. Schlaikjer 



With Five Pla 



N 



CAMBRIDGE^ MASS., U. S. A.: 
PRINTED FOR THE MUSEUM 

June, 1932 



No. 11. — The Osteology of Mesohipptis barboiiri 

By Erich M. Schlaikjer 



Introduction 

The subject of this paper is the detailed description of a new and 
unusually complete Alcsohippus ^ skeleton from the White River 
formation of South Dakota. This skeleton was collected during the 
summer of 192S by a fossil-collecting party, under the direction of the 
writer, sent out by the IVIuseum of Comparative Zoology at Harvard 
Universitv. 

To my knowledge there have been twenty species named of the 
genus Mesohippus. Many of these species, if not most of them, were 
based on very fragmentary material, and it is doubtful if all of them 
are valid. This, however, is not a subject to be discussed in these 
pages. The only detailed osteological description of MrsoJiippiis is 
one by Professor W, B. Scott of Princeton. Until more complete 
skeletal material is found, it will be almost impossible to determine 
w^hich of the less important osteological characters of Mesohippus 
are specific and which are indi\Tdual variations. For this reason it 
does not seem advisable to make a detailed comparison of the charac- 
ters found in the skeleton described here with the characters found in 
M. hairdi. I have chosen to make this paper primarily one of descrip- 
tion, and wish to point out that the specimen herein described presents 
a number of striking characteristics different from those seen in 

ippus as previously known. 

A summary of the outstanding characteristics of this skeleton is 
as follows : (1 ) strikingly developed alveolar portion of the pre maxillary ; 
(2) anterior region of nasals narrow; (3) prominent facial crest on 





ary; (4) large, deeply cupped incisors; (5) premolar-molar series 
especially long in proportion to skull's length; (6) teeth crowns very 
tall; (7) incisor alveolus of mandible broadly expanded; (8) inferior 
incisors large and with tall crowns; (9) inferior canine very incisi- 
form and set close against I3; (10) inferior premolars and molars 
present only minute indication of external cinguH; (11) the posterior 
dorsals and the lumbars form no curvature of the vertebral column 
when articulated; (12) scapula small; (13) humerus very short in 
proportion to length of metacarpal III; (14) metacarpal V a mere 

i Proc. N. E. Z06I. Club, 12, 1031, pp. 3o-3G. 



392 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY 



nodule of bone; (15) lateral and medial digits reduced so they were 
non-functional in standing position; (16) femur very short in propor- 
tion to length of metatarsal III. 

It is evident that this specimen deserved a new specific name, and I 
therefore proposed that it be called Mcsohipiyus barboiiri in recogni- 
tion of Dr. Thomas Barbour's sincere interest in enlarging the verte- 
brate fossil collection at the Museum of Comparative Zoology. 

There can be no mistake about the geological horizon in which the 
specimen was found. It was collected from the White River formation 
in the Big White River Bad Lands, Washington County, 
Dakota, and was found in the upper concretionary formation which 
lies in the upper middle Oreodon zone. 

I owe mv eratitude to Dr. C. C. Mook and to the other members 




'*./ & 



of the Department of Vertebrate Palaeontology in the American Mu- 
seum of Natural History for the permission to study their splendid 
horse material; also to Dr. W. K. Gregory of that institution for his 
kind and ever helpful suggestions. 



Detailed Osteological Description 

In general, the portions of the skeleton which have not been pre- 
served are the following: the anterior region of the nasals; right I^"^ 
and left P; the third, fourth and fifth cervicals; the twelfth dorsal; 
the posterior part of the sacrum and all the caudals; a large number 
of the neural spines ; the pelvis ; most of the left hind limb ; most of the 
ribs; and the sternum. All the other bones of the skeleton are beauti- 
fully preserved, with practically no distortion. 



Upper Dcniition 

The dentition of the specimen is unusually well preserved. Incisors 
two and three on the left side are present. The others are missing. 
They are exceptionally large. The crowns are especially long, are 
deeply cupped, and occupy a vertical position. The roots slope up- 
ward and backward. The crowns of both canines are missing, but the 
prominent roots indicate that they were quite large and were not 
erect. P^ is large and has a strong internal cingulum. It was func- 
tional. The crowns of P^ to M^ are exceptionally tall. P- is smaller 
than any of the remaining teeth. The parastyle is little developed, 
though the tooth is not quadrate in outline. The protoloph is con- 
tinuous with the ectoloph. The metaconule and the hypocone are 



schlaikjer: osteology of mesohippus barbouri 393 



united. The internal cingulum is just slightly developed. The anterior 
cingulum is minute. The crown is not greatly worn. P^ and P"^ are 
very molar-like, and are rather quadrate in outline. There is no indica- 
tion of an internal cingulum on these two teeth. The anterior cingulum 
is more pronounced on P*^ than on P^. The hypostyle is rather promi- 
nent and is somewhat triangular in shape. P^ is distinctly larger than 
P^ and sliditlv larger than INI^ M^"^ are rhomboidal in outline. 



t> 1/ O 



INI^ is less so than the others. There is a prominent oval-shaped 
hypostyle on all three molars. There is no indication of an internal 
cingulum on M^ and ^I", and there is only a slight indication of one 
on jVP. The protoloph and metaloph are rather oblique to the ectoloph 
on P^ to M^. The metaloph is not continuous with the ectoloph in any 
of the premolars or molars. 



The Skull 

The skull is beautifully preserved. There is almost no distortion. 
The only missing parts are: right V'^ and left P; the anterior half 
of left P^; the posterior half of right P^; a small portion of left P'-; the 
anterior part of the nasals; the distal portion of the right paramastoid 
process ; the left tympanic bulla ; and a chip is missing from the anterior 
of the right facial crest. A portion of the maxillaries and premaxillaries 
is chipped away, and a very fine cast of the interior of the skull is 
exposed in this region. 

The occipital condyles are proportionately narrow and are oblique 
in position. The dorsal parts of the articular faces terminate in rather 
deep kidney-shaped depressions, forming a sort of constriction just 
above the foramen magnum. The foramen magnum is almost circular 
in outline, and in the median dorsal position a slight groove extends 
upward for a short distance along the external surface of the occipital. 
Ventrally the condyles are separated by a narrow, shallow groove. 
Near the anterior border of the basi-occipital are two slightly 
rugosities separated by the median line. The paramastoid process 
is slender^ somewhat compressed and extends quite far below the level 
of the condyles. Its position is almost at right angles to the base line 
of the skull (a line extending from the optic foramen to the dorsal 
border of the foramen magnum) . The distal portion is curved slightly 
inward. The supraoccipital is rafher narrow and extends a moderate 
distance on the roof of the cranium. A slight median ridge, with a 
shallow depression on either side extends along the median line of the 
upper half of the supraoccipital. The lambdoid crest is rugose. The 
squamosal is large, and extends upward far enough to form the wall of 





bulletin: museum of comparative zoology 



the widest portion of the brain. The zygomatic portion extends for- 
ward forming a small part of the inferior border of the orbit. The 
articular surface of the glenoid cavity is concave from side to side and 
convex antero-posteriorly; more so in the central portion than exte- 
riorly, where it is broad and somewhat flattened. The post glenoid 
process is high and massive. It is somewhat flattened antero-posteri- 
orlv, and the intero-anterior portion is articular. 




mastoid process is not unusually large. The bulla is not coossi- 
fied with the tympanic, but is held firmly to it by the hyoid process 
which is deeply imbedded in a groove in the bulla. The anterior border 
of the external acoustic meatus is quite heavy. 

The basisphenoid is arched posteriorly and becomes flattened as it 
extends under the vomer. The pterygoid process extends downward 
and forward and is quite pronounced. The alisphenoid extends later- 
ally and dorsally from the basisphenoid at a quite gentle slope along 
the base of the brain. The anterior portion then bends upward and 
forward rather sharply and continues to about half the heiglit of the 
squamosal. 

In front of the ascending portion of the alisphenoid is the ethmoid 
bone. It is narrow and lies in a vertical position. The posterior border 
extends the length of the ascending alisphenoid and then continues 
upward and forward meeting the anterior border, togetlier with which 
it forms a pointed projection under the postorbital process. 

The lachrymal bone forms the greater part of the anterior portion 
of the orbit. The oval-shaped lachrymal fossa is large and is situated 
near the border of the orbit. There is a deep antorbital depression 
centrally located on the facial part of the lachrymal. 

The jugal bone forms the lower border of the orbit. Superiorly the 
maxillo-jugal suture is vertical, and inferiorly it extends downward 
and backward. The lower region of the jugal presents a prominent 
facial crest which is continuous in front with a similar crest on the 
maxillary. 

The parietals form the greater portion of the cranial roof. In each 
the posterior width is one-fourth narrower than the anterior width. 
The sagittal crest is low and in front it separates into two ridge-like 
lines which curve outward and forward to meet the frontal crests. 



Lying bluntly wedged between the foremost parts of the parietals 
are the frontal bones which are prominently convex in this region. 
The area just behind the postorbital processes is comparatively wide, 
indicating a degree of intelligence which perhaps surpassed that of the 
near Oligocene relatives of this animal. The postorbital process is 



schlaikjer: osteology of mesohippus barbouri 395 



moderately long, extending downward two-thirds the distance to the 
zygomatic arch. Just in front of the process is a deep notch which is 
preceded by a prominent tubercle. The anterior portion of the frontals 
is flattened; and at the median line they extend only slightly between 
the nasals. 

The nasal bones are triangular in outline. Posteriorly they are 
flattened. The anterior portion is tranversely convex and exceptionally 
narrow. 

The alveolar portion of the premaxillaries is strikingly developed for 
Mesohippus. The transverse measurement between the alveoli of 
incisors three is 22 mm. The vertical diameter is also comparatively 
large. At the median line there is evidence of a foramen incisivum. 
The diastema between incisor three and the canine measures one- 
fourth the distance of the canine-premolar diastema. The ascending 
portion of the premaxillary is quite oblique in position. The upper- 
most portion is missing and in restoring I have tried to follow out the 
curvature of the parts that have been preserved. The position of the 
premaxillaries and the nasals seems to indicate that the anterior nares 
w^ere quite high, and were broader below^ than above. It is difficult to 
determine the exact extent of the palatine plates of the premaxillaries 
as they are not entirely preserved. 

The facial portion of the maxillary is somewhat convex 
rather concave above; particularly in the anterior region. 
lary portion of the facial crest stands out considerably from the tooth 
row and lies above the posterior half of the first molar. It has the form 
of a prominent elongated tubercle. The infraorbital foramen is situated 
twenty -six millimeters in front of the orbit and eighteen millimeters 
above the front of premolar four. The diastema between the canine 
and the. first premolar is unusually short. There is a rather abrupt 
constriction of the maxillarics at about the middle of the diastema, 
although this region is comparatively broad. The palatine surface is 
transversely convex and is slightly raised along the median suture. 
The alveolar tuberosity extends a good distance behind the last molar 
and is sc^uare in outline. 

The palatines form the anterior margin and the lateral walls of the 
posterior nares. They do not greatly constrict the opening posteriorly. 
The inferior portion of the wall ot the orbit is formed by the palatine 
bone and it extends forward to make contact with the lachrymal. 

The vomer is long and narrow and is slightly constricted where it 
articulates with the pterygoids. 




396 bulletin: museum of comparative zoology 



The Sktdl in General 

The broad diameter across the molar region, the uniformity in width 
throughout the length of the zygomatic arches, and the compara- 
tively expanded cerebral region give the skull a broad massive appear- 
ance. The area anterior to the frontals is constricted though it is 
relatively long. The great development of the pre maxillary alveolar 
region is certainly an advanced characteristic and is unusual in the 
genus MesoMpjms. It is therefore one of the striking features of the 
skull. The large cupped incisors, the tall crowns of the premolars 
and molars, and the comparatively long tooth series are additional 
characteristics of the skull showing advancement at this stage of the 
evolution of the horse. 



Lower Dentition 

Right Pi and P2, and the anterior portion of left P3 are missing; 
otherv\ise the lower dentition is complete. The incisors are very large 
and the crowns are exceptionally long. I3 is slightly smaller then Ii 
and I2. Each has an internal ^^cingule" (a term that I suggest to de- 
scribe a flattened cone-like cusp occupying the position of a cingulum) 
which is situated outwardly from the median line. A quite prominent 
ridge extends from the outer rim down to the inner base of the cingule. 
There is a pocket-like depression on either side of the ridge. The 
outer depression is slightly deeper than the inner onie. These depres- 
sions are deepest in I3. 

The canine is set closely against 1 3 and is very incisiform. A rather 
heavy ridge extends from the outer rim to the internal base, and has 
a slight depression on each side. The posterior depression is a little 
more pronounced than the anterior one. 

Pi is minute and is set close to P2. The tooth is compressed laterally. 
There is a shallow depression in the anterior internal portion of the 
crown, and the posterior portion presents a V-shaped ravine at the base 
of which is a well defined entoconid. The tooth, therefore, has practi- 
cally the same form as P2 though it is very much smaller. It was not 
functional at the time of the animal's death. P2 is incompletely molar- 
form, the metaconid not being developed. P4 is the largest tooth of 
the Pi — IMo series. M3 has a well developed, deeply cupped hypo- 
conulid, and is therefore the longest of any of the inferior teeth. P4 
has the greatest breadth. There is only the very slightest indication 
of an external cingulum on any of the inferior molars. 



schlaikjer: osteology of mesohippus barbouri 397 



The Mandible 
The horizontal ramus is elongated. It tapers forward and is bicon- 



vex in transverse vertical section. The two rami are firmly fused at 
the symphysis, which is somewhat shortened. There is a rather sharp 
constriction at the diastema, but this region is comparatively broad. 
The incisor alveolus is broadly expanded. The symphysial region is 
extended upward and forward at a rather sharp angle from the remain- 
ing portion of the horizontal ramus. This shortening and angulation 
of the symphysial region was probably necessary to supply additional 
strength for the support of the large incisors. The ventral border of 
the ramus posterior to the symphysis is nearly straight . At its posterior 
part the vascular impression is fairly well marked. The angle is 
rounded. Its posterior border is thickened and slightly medially 



curved. The foramen mandibulare is large and is situated on a level 
with the base of the tooth row. The condvle is considerablv elevated 
above the level of the molars. It is extended transversely and occupies 
an oblique position, the inner portion being directed backward and 




nward. The coronoid process extends high above the condyle. It 
is thin transverselv, curved backward and slightlv inward. 



Basal length 



Skull Mcasnrcvients 

mm. 

Condylo-basal length 176. 

Zygomatic breadth 85. 

Greatest breadth across squamosals on molar arches .... 82. 

Greatest breadth across squamosals on cranium 52. 

Breadth on upper rim of post glenoid notch 51.5 

Interorbital breadth across frontals 45. 

Distance from anterior rim of orbit to pmx. (I alveoli) . . , . 88. 

Distance from anterior rim of orbit to supraorbital crest . . . 110. 

Distance from mandibular condyle to pmx 146. 

Breadth of muzzle at anterior root of M^ 60.5 

Distance from anterior end of internal nares to pmx. (I alveoli) SO., 

Distance from anterior end of internal nares to condvles . . . 96. 

Greatest width of condvles ..." 30.5 

Distance from ventral portion of exocc. to top of Supraocc. . . 48. 

Width of premaxillaries at posterior of P alveoli 

Breadth across P^ to P^ at center of crown . 

Breadth across P'* to P^ at center of crown 57. 





39S 



* « 



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Breadth across M^ to M^ at center of crowni 
Breadth across ]\P to j\P at center of crown 
Length of diastema (P to P^) external . . . 



Molar series on alveoli (P^ to IVP) external . 
Distance from P" to P^ on alveoli (external) 
Distance from M^ to ]VP on alveoli (external) 
Height of crown of P at center of tooth (external) 
Lateral distance across P ..... 
Fore and aft distance at center of I- 

Height of crown of P 

Lateral distance across P 

Fore and aft distance at center of P 



Height of crown of P^ at highest point (external) 
Height of crown of P- on paracone (external) 



Brea 




of P- at center of crown 



Length of P- at center of crown 

Height of crown of P^ on paracone (external) 
Height of crown of P^, IM^' ^' ^ on paracone (extern 
Breadth of P^ at center of crown ....... 

Length of P^ at center of crown ....... 

Breadth of ]M^ at center of crown ....... 

Length of INI^ at center of crown 
Breadth of ^P at center of crown 
Length of ]\P at center of crown 
Greatest breadth of 




Greatest length of ]NP 



al) 



mm. 
59. 

22. 

76. 
35. 
35. 
5.5 

7. 

5.5 

5.5 

7. 

5.5 

5.5 

11.5 
10. 
9.5 
10.5 
14.5 
12.5 
14.5 
12.5 
14. 
11. 
15.5 
13. 



The Vcrtchral Column 

Atlas: This vertebra is broad in proportion to its length. The 
anterior articular cavities are deep. They almost meet ventrally, but 
dorsally they are widely separated by a broad notch. The neural arch 
is broad and high. Medially there is a prominent, sharply convex 
region. There is only a slight indication of a ridge above the inter- 
vertebral foramen. The ventral arch is more massive, narrower, and 
shorter than the dorsal arch. The surface is flattened. The hypa- 
pophysial tubercle is posterior in position. It is fairly large and very 
prominent. The posterior articular faces are high. They are widely 
separated al)Ove and are nearly confluent below. The surfaces are 
slightly convex and the inner borders are rounded off. The wing is 



schlaikjer: osteology of mesohippus barbouri 




somewhat short, but fore and aft it is quite extended. The dorsal 
surface is sUghtly concave. At the anterior base is the spinal nerve 
notch which is fairly deep. The foramen trans versarium is situated 
on the border of the posterior root of the wing where it meets the 
articular face. The fossa atlantis is large, rounded and deep. The 
posterior border is narrow. 

Axis: Only the anterior region of the axis is preserved. The odontoid 
process is broad and is turned upward at the end. It has a low convex 
ventral surface; the dorsal surface presents a fairly prominent median 
ridge with a depression on either side. The anterior articular processes 
are low, broad, and are confluent with the lateral margins of the 
odontoid process. The articular surfaces are slightly concave trans- 
versely and markedly convex dorso-ventrally. 

CcTvicah: The third, fourth, and fifth cervicals are absent. The 
sixth cervical vertebra is deep and massive. The neural spine is in- 
complete, but the portion present indicates that it was scarcely more 
than a prominent ridge. The prezygapophyses are missing. The post- 
zygapophyses project slightly behind the neural spine. The transverse 
process is straight, short, and heavy. The front part of the inferior 
lamella is not preserved. Posteriorly the lamella extends far downward 
and slightly outward; its inferior border is flattened. Below the pos- 
terior end of the foramen transversarium is a rather large fossa. The 
head of the centrum is oval-shaped; it is wider and more flattened 
above than below. 

The seventh cervical is considerably shorter, but is wider than the 
sixth. The neural spine is very short, measuring only eighteen milli- 



meters above the dorsal border of the neural canal. The prezygapo- 
physes are wider than the postzygapophyses; the latter extend slightly 
behind the neural spine. The transverse process is moderately long. 
The distal border is bent upward and is thickened; anteriorly it is 
bent downward and is thin. The posterior part of the centrum is wide 

and flat. 

Dorsals: The exact number of dorsal vertebrae is uncertain. The 
matrix containing the specimen w^as broken just behind the eleventh 
dorsal. I have reckoned the twelfth to be missing. The remaining five 



were in continuous series; thus making seventeen dorsals in the 
mounted specimen. The first of" the series resembles the seventh 
cervical in some respects. The centrum is flattened, though it is 
shorter and the median ventral portion is heavier. The transverse 
process is even longer and is formed into a deep semi-circular facet 
for the tubercle of the first rib. The prezygapophyses are wnder than 



400 bulletin: museum of comparative zoology 





the postzygapopliyses, and the latter extend only slightly behind the 
neural spine. The neural spine is thin in front and broad behind; gi\Hng 
it a triangular shape when viewed from above. There is a median 
ridge on its posterior border. The succeeding dorsals show modifica- 
tions of these characters. The neural spine is heaviest on the second 

; from there on back to the thirteenth it gradually becomes 
Hghter. Only a small part is missing from the end of the spine of the 
second dorsal. The tips are missing from the next four, and the greater 
portion of each spine of the others has been lost. In restoring the 
missing ones I have been guided only by what remains of each. The 
semi-circular facets on the transverse processes are gradually reduced 
so that they are flattened on the sixth to the last. The zygapophyses 
on the third to the sixteenth are developed on the neural arch. The 
last dorsal is the only one which has a median ventral keel; the keel 
shows greater development in front than behind. The vertebra also 
presents a quite prominent elongated tubercle on the side of the 
centrum behind the rib facet. From the fifteenth to the seventeenth 
the length of the centrum gradually increases. 

Lumhars: The lumbar vertebrae Avere discontinuous behind the 
third of the series. Two lumbar centra were associated with the speci- 
men, and I have considered them to be the fourth and fifth. The 
sixth and seventh w^ere continuous with the sacrum. All of the lumbars 
are fairly completely preserved. A large amount of each neural spine 
is preserved, except on four and five w^here only the centra remain. 
The seventh is the only one which retains the greater portion of its 
transverse processes. The centra of the first four are quite elliptical in 
cross-section and have a distinct ventral crest. From the fifth to the 
last the centra become more flattened and the ventral crest disappears. 
The basal portions preserved of the transverse processes indicate that 
they were moderately massive and that they extended outward and 
slightly downward. On the last three they project moderately forward. 
The transverse process of the last lumbar has a thickened oval-shaped 
facet for articulation with the wing of the sacrum. 

Sacnwi: Only the first of the sacral vertebrae is preserved. The 
centrum is considerably flattened. The wing is massive and curves 
slightly dow^nw^ard. On its anterior border is a convex surface which 
fits into the facet on the transverse process of the last lumbar. On the 
postero-dorsal surface of the w^ing is the elongated, concave, rugose, 
articular surface for articulation with the ilium, 

Candals: None of the caudal vertebrae are preserv 




SCHLAIKJER: osteology of MESOHIPPUS BARBOUR! 




Vertebral Column in General 

The vertebral column presents a number of interesting features, 
such as: the shape of the neural arch of the atlas and the position of 
the foramen transversarium on that vertebra; the shortness of the 
neural spines on the sixth and seventh cervicals; the absence of a 
ventral keel on all the dorsals except the last; and many other similar 
characters. But perhaps most striking of all is the lack of curvature 
of the column when the vertebrae are articulated. The posterior 
cervicals and the anterior dorsals form a downward curving of the 
column but the posterior dorsals together with the lumbars almost 
form a horizontal line. This certainly seems to be an advanced char- 



acteristic. 



The Ribs 



Most of the ribs are lacking. The first three and the fifth on the left 
side and the second and third on the right were found in position and 
are complete. They are comparatively short; the first is rounded and 
slender, but the others are broad and very flattened. The proximal 
ends and various parts of a number of others are present. None of the 
sternum was preserved but the increase in length of the first five ribs 
seems to indicate that it must have occupied a position oblique to the 
long axis of the horse^s body. 



Bones of the Fore Limh 

The greater portion of the left scapula and a good deal of the right 
is preserved; all the remaining bones of the fore limbs, including a 
number of the sesamoids, are complete and Avere articulated. 

Scapula: The glenoid cavity is slightly longer than broad, and it is 
fairly shallow. The coracoid process is large and massive; it is some- 
what recurved at the distal end. The neck is long and exceptionally 
slender. The anterior border is moderatelv thick near the coracoid 
process. The posterior border on the lower half of the scapula is con- 



siderably thickened. The distal part of the scapula is erect, but the 
proximal region is curved in, giving it the appearance of a bow when 
viewed from behind or in front. The spine is tall and is bent sharply 
backwards. 

Humerus: This bone is proportionately very short for Mcsohippns. 
The head is large, strongly convex and is rather circular in outline. 
The external margin is considerably lower than the internal. This, 
along with the strong convexity of the surface, perhaps indicates that 



402 bulletin: museum of comparative zoology 



there was a good deal of lateral swing to the foreUmb during locomo- 
tion. The anterior portion is large and overhangs the l^icipital groove. 
The more lateral part is well developed and its outer surface presents 
a large rugose triangular area for the attachment of the infraspinatus 
muscle. At the bottom of the bicipital groove there is a slight mdica- 
tion of a tubercle. The inner tuberosity is large and has a flattened 
dorsal surface. The shaft is comparatively slender and is twisted; it is 
irregularly oval-shaped. The deltoid tuberosity is prominent and 
rugose. The distal end is somewhat broadly expanded; the trochlea 
is oblique to the long axis of the shaft and the epicondyles are fairly 
well developed. 

Radius: The radius is comparatively long. The shaft is gently curved 
forward and is flattened antero-posteriorly; more so medially than 
laterally. It is rather expanded at both ends. The interior facet of 
the proximal end is fairly shallow and is larger than the narrow median 
facet. The external facet is large and flattened; it extends outward and 
upward. The distal end is massive. The grooves for the common and 
lateral extensor tendons are well marked. On the inner anterior surface 
there is a shallow oblique groove for the tendon of the extensor carpi 
obliquus muscle. The scaphoid facet is quadrilateral and is concavo- 
convex from before backwards. The lunar facet is similar in shape, 
though with less convexity and with more transverse expansion. 

Ulna: The ulna is much reduced. The olecranon is high and massive. 
The medial side is concave, the lateral is convex. The top presents a 
heavy rounded tubercle situated posteriorly. There is a prominent 
ridge extending from the tubercle with a fairly large, flattened face 

. The processus anconaeus is very prominent. The semi- 
lunar notch is rather deep and its surface is convex transversely. The 
distal end of the ulna is not fused with the radius. Its articular face 
is convex and bears a postero-internal groove. 

Carpus: The scaphoid is deep, somewhat laterally compressed and 
is rather six-sided in appearance. The anterior portion of the dorsal 
surface is convex and the posterior is concave. There is an upper and a 
lower facet for the articulation with the lunar. On the distal surface 
the magnum articulation is slightly convex. Directly behind is the 
facet for the trapezoid which is rounded and concave. There is a 
minute facet behind this for the trapezium. 

The lunar is proportionally long; it is wider in front than behind. 
The dorsal surface is saddle-shaped. Laterally the lunar is in contact 
with the scaphoid at the superior and inferior margins. It is in con- 
tinuous contact with the cuneiform. The ventral surface is narrower 




schlaikjer: osteology of mesohippus barbouri 403 



than the dorsal. The articulation with the magnum is only slightly 
wider than the unciform articulation. There is a tubercle on the 
ventro-posterior surface. 

The cuneiform is narrow above and broad below. On the anterior 
internal proximal surface there is a small triangular facet for the 
radius. The ulna facet is concave and is higher in front than behind. 
On the distal surface the unciform is concave and is triangular in 
outline. The pisiform facet is oblique in position. 

The pisiform is deep, somewhat flattened and moderately long. The 
internal surface is concave and smooth. The external surface is convex 
and rugose. Its anterior part bears a shallow oblique groove for the 
long tendon of the ulnaris lateralis. The facet for the ulna is round and 
convex. The cuneiform facet is antero-inferiorly situated. It is elon- 
gated and convex, and is separated from the ulna facet. 

The trapezium is not preserved. 

The trapezoid is low and rather wedge-shaped, behig narrow in 
front and broad behind. The proximal surface is convex and continu- 
ous on the volar surface. The distal surface is flattened. The articula- 
tion for metacarpal II is rather triangular in outline. There is a narrow 
elongated articular surface on the ventral border for articulation with 
the magnum. 

The magnum is the broadest of any of the distal carpal bones. On 
the proximal surface, the facet for the scaphoid is twice as broad as 
the facet for the lunar. The distal surface is broadly triangular in 
outline and is almost flat. 

# 

The unciform is somewhat shortened, its greatest length being only 
one-fourth greater than that of the magnum. It is rather broad. On 
the proximal surface the lunar facet is fairly large and has a dorsal 




position. The cuneiform is slightly convex. On the distal sur- 
face, the facet for metacarpal IV is slightly concavo-convex from 
before backwards. The facet for metacarpal III is ventro-laterally 
situated. There is a small facet for metacarpal V. 

Metacarpus: Metacarpal III is long in proportion to the other bones 
of the limb. The shaft is rather semi-elliptical. The anterior surface is 
transversely convex. The posterior surface is slightly convex from 
side to side and there is a medial shallow groove running longitudinally. 
The proximal surface is flattened ^anteriorly, and it bends down to a 
certain extent posteriorly. The unciform facet is oblique in position. 
The distal end is expanded and thickened. The carina is prominent 
and extends slightly anterior to the center of the long axis of the bone. 
On either side of the articular surface is a small fossa above which is a 
prominent tubercle. 



404 bulletin: museum of comparative zoology 



The lateral metacarpals are shorter, thinner, and more compressed 
than metacarpal III. Metacarpal IV is the shorter of the two. The 
distal extremities are rounded and the carinae are mostly confined to 
the posterior. 

Metacarpal V is minutely represented — its greatest length being 
only eleven millimeters. The head is half the size of that of metacarpal 
IV. The entire metacarpal amounts to nothing more than a nodule of 
bone. 

Phalanges: The phalanges of the lateral digits are short and stubby; 
the unguals are short, narrow, and pointed. Those of the middle digit 
are very much larger. The first phalanx is long, broad and thickened 
above, and more narrow and compressed below. The second phalanx is 
much broader than it is long. It is not quite two-thirds as long as the 
first and is much compressed antero-posteriorly. The ungual phalanx 
is somewhat longer than it is broad. The dorsal surface is strongly 
convex. The ventral surface is prominent but not greatly extended 
posteriorly. The distal border is notched and a V-shaped groove 
extends upward from the notch a short distance along the dorsal 
surface. 



Measurements 



mm. 




Length of scapula 

Fore and aft diameter of glenoid cavity 19.5 

Transverse diameter of glenoid cavity 17. 

Distance from coronoid process to posterior rim of glenoid 



cavity 



27 



Narrowest fore and aft distance of neck of scapula 13.5 

Length of humerus, external tubercle to posterior trochlea . . 123. 
Length of humerus, head to posterior trochlea 117. 



Shortest width of shaft 



11. 



Shortest fore and aft distance of shaft 14. 

W idth of distal end of humerus on lateral ligament depressions . 23.5 



Greatest length of ulna . . . 
Length of radius on inner side 



166. 
130. 



Greatestwidthof proximal end of radius 24.5 

Greatest width of distal end of radius 21. 

Q 

10 ^ 



Width at center of radius shaft 
Greatest height of lunar . . . 
Greatest height of magnum . 
Width of magnum (anterior) . 



schlaikjer: osteology of mesohippus barbouri 



405 



mm 



Greatest length of metacarpal III . 

Greatest width at proximal end of metacr4rpal III 
Greatest width at distal end of metacarpal III . 

Greatest length of metacarpal II 

Greatest length of metacarpal IV 

Greatest length of metacarpal V ....... 

Lateral median length of first phalanx, 3d digit . 
Lateral median length of second phalanx, 3d digit 
Median dorsal length of ungual phalanx, 3d digit . 
Width of ungual phalanx (across angles) 3d digit . 



102. 

14.5' 
95.5 
9L 
11. 
14, 
9- 

17.5 
15. 



Bones of the Hind Limb 

Practically all of the right hind limb is preserved. The pelvis and the 
left hind limb, with the exception of a part of the proximal end of the 
femur and most of the astragalus, are missing. 

Fcmvr: In proportion to the length of the metatarsals the femur is 
very short. The proximal end is large. The head is practically hemi- 
spherical and is directed upward, forward and inward. The notch for 
the accessory and round Hgaments is deep and broad. The great tro- 
chanter is very massive. The anterior part is low and rugose; a shallow^ 
notch separates it from the posterior part which stands high above the 
head and is mediallv curved at the end. The trochanteric fossa is 



elongated and deep. The second trochanter is prominent; the distal 
end of the third is missing but the region preserved indicates that it 
was weW developed and was curved forward. 

The shaft is stout. Just below the third trochanter it is laterally 
compressed and deep; it then becomes circular, and expands and 
deepens at the distal extremity. 

The distal end is expanded both transversely and antero-posteriorly. 
The trochlea is broad and slightly oblique to the long axis of the shaft; 
the medial ridge is heavier and more extensive than the lateral. The 
condyles are large and extend rather far posteriorly; the lateral is 
much larger than the medial. The medial and lateral epicondyles are 
fairly prominent; the supracondyloid fossa is elongated and deep. 

Tibia: This bone is long and rather slender. The proximal end is. 
large and triangular in general outline when viewed from above.. 
The condyles are saddle-shaped, and the lateral is broader than the 
medial. The intercondyloid eminence is prominent; the popliteal notch 
is broad and fairly deep. The sulcus muscularis is semicircular and 



406 bulletin: museum of comparative zoology 



deep. The crest is very prominent and rugose, and occupies the upper 
third of the shaft. The distal end is narrow but massive; the medial 
portion is larger than the lateral. The articular facets for the astrag- 
alus are oblique to the long axis; the medial is narrower and deeper 
than the lateral. 

Fibula: Both ends and a part of the distal region of the shaft are 
preserved. The proximal end is large; it is flattened transversely, and 
the anterior and posterior borders are rounded. The distal end and a 
part of the lower end of the shaft are coossified with the tibia. The end 
is expanded and the calcanear facet is large and flattened. 

Patella: The patella is cubical in outline with a pointed postero- 
distal end. The anterior surface is strongly convex and rugose; the 
posterior presents two articular surfaces, of which the medial is slightly 
larger. The medial border is short and slightly convex; the lateral 
border is long and straight. The base is transversely convex and is 
slightly concave in the antero-posterior diameter. 

Tarsus: The calcaneum is proportionately short; it is transversely 
compressed. The distal end of the body is thickened, rounded and 
rugose. The plantar border is straight and slightly expanded at each 
end. The posterior half of the dorsal border is straight. The cochlear Is 
process is prominent and is expanded laterally. The sustentaculum is 
large and massive. The cuboid facet is somewhat narrowed and is 
slightly concave. 

The trochlea of the astragalus is oblique in position; the groove is 
narrow and deep. The distal surface is transversely concave and is 
convex antero-posteriorly. There is a small cuV)oid facet. The distal 
part of the medial surface bears a large tuberosity. 

The cuboid is deep, transversely compressed and is not in contact 
with metatarsal III. The navicular is low and broad. The meso- and 
ento-cuneiforms are coalesced. The en to-cuneiform is rounded pos- 
teriorly and is in contact with the cuboid; it is deeper than the meso- 
cuneiform. The ento-cuneiform is broad and rather low. 

Metatarsus: Metatai'sal III is proportionately long and fairly stout. 
The proximal end is narrow and deep and does not come into contact 
with the cuboid. The distal end is broadly expanded but massive. 
On the dorsal side above the articular surface is a transverselv elon- 
gated shallow^ pit. The lateral tubercles are very prominent. Meta- 
tarsals II and IV are much shorter than metatarsal III and are much 
lighter in construction. 

Phalanges: The phalanges of the hind foot are practically the same 
as those of the fore foot except they are more massive; the portions 



schlatkjer: osteology of mesohippus barbouri 407 



preserved of the lateral and medial digits seem to indicate that they 
were shghtly shorter. 



Measurements 



mm. 




122. 



Greatest length of femur 

Length of femur from head to internal trochlear ridge .... 157. 
Length of femur, center of 3d trochanter, to external trochlear 

Shortest transverse measurement of femur shaft 13. 

Distance across condyles of femur 33. 

Length of tibia from medial condyle to internal malleolus . . 182. 

X-^Cll^til 01 TlDllicl - ♦..•...* 

Metatarsal III (maximum length) 117. 

Maximum width, proximally, of metatarsal III 13. 

Maximum width, distally, of metatarsal III 15. 

113 

109 
1 ^ 




Maximum length of metatarsal II 

Maximum length of metatarsal IV 

Length of 1st phalanx, 3d digit 

Lengthof 2d phalanx, 3d digit, at center of side 10. 

Lengthof ungual phalanx, 3d digit, dorsal 19. 



llie Limbs in General 

Perhaps the most striking feature of the limbs is the shortness of 
the proximal elements as compared with the proportionately long 
metacarpals and metatarsals. It is a well known fact that lengthening 
of both proximal and distal limb elements creates a long stride in 
locomotion but retards speed. On the other hand, if only the distal 



elements of the limb are lengthened and if the proximal elements are 



shortened then both the stride and the motion of the limb are increased, 
thus giving the animal greater speed. According to the accompanymg 
table, showing the metacarpo-humerus and metatarso-femur ratios, 
it seems evident that this specimen was proportionately more swift 
in running than were his near Oligocene relatives. It is indeed inter- 
esting to note that the speed of this little horse was, proportionately, 
perhaps even greater than that of the upper Oligocene ^Vstilt-footed'' 
Mesohippus grallipes. 



408 



bulletin: museum of comparative zoology 



Table showing a comparison of limb ratios of M. barbouri with 
those of other species of Mesohipjms, 





Humerus 
head-tr. 


Metar. HI 
length 


Ratio 


Femur max. 
lenfjth 


Metat. Ill 


1 

Ratio 


M. barbouri 




*> 




■ 






M. C.Z. 


117. 


102. 


.872 


174. 


117. 


.672 


17641 














M. bairdd 














M. C. Z. 


128. 


93. 


.727 


' 197.5 


- 115. 


.582 


10682 














M. bairdi \ 














A. M. N. H. 


137. 


88. 


.642 


220.5 


115. 


.522 


1477 






» 


+ 






M. grallipes 






j 








Princeton 














museum 


176. 


148. 


.841 


256. ± 


180. 


.703 


10729 
















Another striking feature of the Hmbs is the reduction of the lateral 
and medial digits so* they were non-functional in standing position. 
This does not mean, however, that they were entirely useless. They 
were probably still used to help keep the feet of the animal from 
miring deeply into the marshy lands on which it probably spent a 
good deal of its roving existence. 

It is hoped that what has been written in the foregoing pages wi 
contribute something to that which is already known of Mesohipinis. 
To say what w^as the exact ancestry of M. barbouri would only be 
hypothetical. I believe, however, that this species represents a pro- 
gressive line of three-toed horses which had its origin from some as 
yet undiscovered genus of horse in middle or late Eocene times; and 
from a species more advanced than that which gave rise to the line 
of Oligocene horses represented by M. bairdi. 



schlaikjer: osteology of mesohippus barbouri 409 



BIBLIOGRAPHY 



Douglas, E. 



1903. New vertebrates from the Montana Tertiary. Ann. Carn. Mus,, 



2, pp. 145-200. 



Fare, M. S. 

1896. Notes on the Obteology of the White River horses. Proc. Amer. 

Phil. Soc, May 15, 35, pp. 147-175. 
Lambe, L. M. 

1905. Fossil horses of the Oligocene of the Cypress Hills, Assiniboia. 

Trans. Royal Soc. of Can., Second Ser., 11, Sec. IV, pp. 43-52, 
pi. 2. 




Lambe, L. 

1905. On the tooth structure of Mesohippus westoni (Cope). Amer. 

Geol., 35, April, pp. 243-245. 
Leidy, Joseph 

1869. Extinct Mammalian Fauna of Dakota and Nebraska. Jour. 

Acad. Nat. Sci,, Phila., 2d ser., 7, pp. 257-330. 
Matthew, W. D. 

1903. Evolution of the horse. Guide Leaflet No. 9, Amer. Mus. Jour., 

3, No. 1, Second ed., 1905. 
1915. Chmate and evolution. Ann. N. Y. Acad. Sci., 24, pp. 171-318. 

33 text figures. 

1926. The evolution of the horse. A record and its interpretation. 

Quart. Review of Biol., 1, No. 2, pp. 139-185. 27 text figures. 

1928. Outline and general principles of the history of life. Univ. of Cal. 

Press. Berkeley, Cal. 253 pages, 21 text figures. (Chapter XX. 
The evolution of the horse, pp. 160-177). 
Matthew^, W. D. and Chubb, S. H. 

1913. Evolution of the horse. Guide leaflet No. 36. Amer. Mus. Nat. 

Hist. Pt. I, pp. 1-35 by W. D. Matthew, evolution of the horse in 
nature. Pt. 2, pp. 39-63 by S. H. Chubb, the horse under domesti- 
cation. Republished 1921 with slight revision. 

OSBORN, H. F. 

1904. New Oligocene horses. Bull. Amer. Mus. Nat. Hist., 20, pp. 167- 

179, pis. Ill, V. 

1918. Equidae of the Oligocene, Miocene, and Pliocene of North America. 

Iconographic revision. Mem. Amer. Mus. Nat. Hist., n.s., 2, 
pp. 1-217, pi. 1-54, and 173 text figures. 

gCHLAIKJER, E. M. 

1931. Description of a new Mesohippus from the White River formation 

of South Dakota. Proc. N. England Zool. Club, 12, pp. 35-36. 
Scott, W. B. 

1891. On the Osteology of Mcsohippvs and Leptomerijx, Jour. Morphol., 

6, pp. 301-400. 




410 bulletin: museum of comparative zoology 



SiXCLAIR, W. J. 




, The faunas of the concretionary zones of the Oreodon beds, White 
River Oligocene. Proc. Amer, Phil. Soc, 63, No. 1, pp. 94-133. 
pL 1 and 10 text figures. 
1925. The mounted skeleton of a new Mesohipjms from the Protoceras 

beds. Proc. Amer. Phil. Soc, 64, pp. 55-63, ph 3. 



EXPLANATION OF PLATES 



4 



PLATE 1 



ScHLAiKJER.— The Osteology of Mesohippus barbouri 



PLATE 1 



Mesohippus harhouri 






^1 SlZ^ft 



BULL. MUS. COMP. ZOOL. 



SCHLAIKJER. MESOHIPPUS. PLATE 1 




PLATE 2 



SCHLAIKJEB.— The Osteology of Mesohippus barbouri 



PLATE 2 



Mesohippus harbouri Schlaikjer 



Skull and jaws, left side; Actual size. 



BULL. MUS. COMP. ZOOL. 



SCHLAIKJER. MESOHIPPUS. PLATE 2 







PLATE 3 



ScHLAiKJEB. — The Osteology of Mesohippus barbouri 



PLATE 3 



Mesohippus barbouri Schlaikjer 
Skull and jaws, palatal view. Two-thirds actual size. 



• i 



BULL. MUS. COMP. ZOOL. 



SCHLAIKJER. MESOHIPPUS. PlATE 3 




PLATE 4 



ScHLAiKJER. — The Osteology of Mesohippus harbour i 



PLATE 4 



Mesohippus harbotiri Schlaikjer 

Vertebral column; Fig. 1, left side; fig. 2, ventral view; fig. 3, dorsal view; 
Reduced 4f times. 



* • 



BULL. MUS. COMP. ZOOL. 



SCHLAIKJER. MESOHIPPUS. PLATE 4 



I 



2 



3 




* .1 



1"^ 1 



^A 



/ 



PLATE 5 



ScBXAiEjEB. — The Osteology of Mesohippus barbouri 



PLATE 6 



Mesohippus barbouri Schlaikjer 
Mounted skeleton, right side. ^ natural size. 



Photographs by Irving Dutcher. 

Skeleton mounted by Charles J. Lang. 

All figures based on the type, M. C. Z. no. 17,641. 



• * 



BULL. MUS. COMP. 200L. 



SCHLAIKJER. MESOHIPPUS. PLATE 5